The present invention relates to a pneumatic tire improved in various tire performance parameters, which is effectively reinforced by incorporating a specific short fiber into a rubber member constituting the tire so as to satisfy the basic requirements for a tire.
As a means for reinforcing the rubber member constituting a tire, it has been a common practice to embed reinforcement cords in the rubber or to use a high-hardness rubber. When the rigidity of the rubber member was uniformly changed to improve a specific tire performance, however, a problem occurred that the other tire performance parameters were lowered.
With respect to a pneumatic radial tire, for example, it has been known that road noise can be reduced by enhancing the rigidity of the shoulder portions near the belt edges. It has therefore been a general practice to insert sheets or fillers made of a high hardness compound under both belt end portions to thereby enhance the rigidity of the shoulder portions. When such sheets or fillers made of a high-hardness compound are used, however, the basic performance parameters such as comfortableness in riding and stability in steering are changed by enhancement in the rigidity of the shoulder portions, even though road noise can be reduced.
Further, attempts have been made to enhance selectively the modulus of a rubber member constituting a tire in a specific direction by incorporating a short fiber made of nylon or the like into the rubber member and orienting it in the specific direction. However, the short fibers of the prior art have failed in attaining such effective reinforcement as to satisfy the basic requirements for a tire.
The first object of the present invention is to provide a pneumatic tire that is so effectively reinforced by incorporating a specific short fiber into a rubber member constituting the tire as to satisfy the basic requirements for a tire.
The second object of the present invention is to provide a pneumatic tire which is reduced in road noise by the use of the above specific short fiber with the basic performance parameters such as comfortableness in riding and stability in steering being kept at high levels.
The third object of the present invention is to provide a pneumatic tire which can exhibit excellent braking performance both on a dry road surface and on a wet road surface by virtue of the use of the above specific short fiber.
The fourth object of the present invention is to provide a pneumatic tire comprising an under tread made of a rubber exhibiting a small energy loss (for lowering rolling resistance), which tire has a reduced nonuniformity of gauge resulting from rubber flow occurring during the molding of the tire with vulcanization and improved stability in steering, by virtue of the use of the above specific short fiber.
The fifth object of the present invention is to provide a pneumatic tire that is reduced in the sidewall gauge by the use of the above specific short fiber with a reduction in the weight without impairing the resistance of the sidewall to external damage.
The sixth object of the present invention is to provide a pneumatic tire that is improved in durability by the use of the above specific short fiber with a decrease in the rubber volume of the bead portions.
The seventh object of the present invention is to provide a pneumatic tire which is reduced in the weight and improved in comfortableness in riding by the use of the above specific short fiber with the stability in steering being kept at a level equivalent or superior to that of a tire provided with steel reinforcements in the neighborhood of the bead fillers.
The pneumatic tire of the present invention achieving the above first object is one comprising a rubber member containing a short fiber (Axe2x80x2) formed by the fibrillation of a short fiber (A) whose cross section takes a sea-island structure essentially composed of at least two polymers.
When the above fibrillated short fiber (Axe2x80x2) is incorporated into the rubber member, the short fiber (Axe2x80x2) enhances the modulus of the member at low elongation by leaps and bounds to exhibit excellent reinforcing effect.
The term xe2x80x9cmodulusxe2x80x9d used in this specification refers to xe2x80x9cmodulus at 20% elongationxe2x80x9d (hereinafter referred to merely as xe2x80x9c20% modulusxe2x80x9d). A tire is generally used in the strain range of 50% or below, so that the characteristics of a rubber at low elongation based on 20% modulus highly correlate with tire performance parameters. Thus, a tire can be reinforced effectively by enhancing the 20% modulus. Accordingly, the reinforcement of a tire with the above short fiber makes it possible to improve various tire performance parameters required depending on the use of the tire.
Such 20% modulus can be determined according to the method of tensile test at low elongations as stipulated in JIS K6301. This method comprises using a test piece having a width of 5 mm, a length of 100 mm, a thickness of 2 mm and a distance of 40 mm between two gage marks, stretching this test piece preliminarily twice at a rate of 45xc2x115 mm/min by 1.5 times the elongation (20%) to be subjected in the test, stretching the resulting test piece by 20% at the same rate as that employed in the preliminary stretching and keeping the same in the state thus stretched, and measuring the load after the lapse of 30 seconds. The modulus at 20% elongation (20% modulus) can be determined by the following formula. The measurement of the load is repeated generally four times and the average of the four values is used.
"sgr"20=F20/S
"sgr"20: modulus at 20% elongation (MPa)
F20: load at 20% elongation (N)
S: sectional area of test piece
A pneumatic tire of the present invention achieving the above second object is a pneumatic radial tire which comprises a carcass layer bridging a pair of, i.e., lefthand and righthand bead portions and a plurality of belt layers lying in the tread portion on the outside of the carcass layer and which further comprises compound sheets lying in both tire widthwise end portions of the belt layers respectively, with the ratio of the tire circumferential modulus (b) of the compound sheets to the tire radial modulus (a) thereof (i.e., b/a ratio) being adjusted to 1.2 or above by incorporating a short fiber (Axe2x80x2) formed by the fibrillation of a short fiber (A) whose cross section takes a sea-island structure essentially composed of at least two polymers into the rubber constituting the compound sheets in a proportion of 0.5 to 15 parts by weight per 100 parts by weight of the rubber and orienting the fibrillated short fiber (Axe2x80x2) in a tire circumferential direction.
Another pneumatic tire of the present invention achieving the above second object is a pneumatic radial tire which comprises a carcass layer bridging a pair of, i.e., lefthand and righthand bead portions and a plurality of belt layers lying in the tread portion on the outside of the carcass layer and which further comprises compound sheets lying in both tire widthwise end portions of the belt layers respectively, with the ratio of the tire circumferential modulus (b) of the compound sheets to the tire radial modulus (a) thereof (i.e., b/a ratio) being adjusted to 1.2 or above by incorporating a short fiber (Axe2x80x2) formed by the fibrillation of a short fiber (A) whose cross section takes a sea-island structure essentially composed of at least two polymers and a composition comprising a short fiber (B) made of a thermoplastic polymer having amido groups in the main chain and a matrix made of a rubber and/or a polyolefin wherein the short fiber (B) is dispersed in the matrix and chemically bonded to the matrix into the rubber constituting the compound sheets in proportions of 0.5 to 10 parts by weight of the fibrillated short fiber (Axe2x80x2) and 1 to 15 parts by weight of the short fiber (B) per 100 parts by weight of the rubber and orienting the fibrillated short fiber (Axe2x80x2) and the short fiber (B) in a tire circumferential direction.
In the above cases wherein such anisotropic rubber compound sheets that the tire circumferential modulus (b) is larger than the tire radial modulus (a) by a specific factor or above are laid in both end portions of the belt layers, the rigidities in the tire width direction and in the tire radial direction (perpendicular to the tire rotating shaft) can be adjusted to levels equivalent to those of the prior art with an enhancement in the tire circumferential rigidity of the shoulder portions near the belt edges. This makes it possible to reduce road noise with the basic performance parameters such as comfortableness in riding and stability in steering being kept at high levels.
A pneumatic tire of the present invention achieving the above third object is one wherein the ratio of the tire circumferential modulus (b) of the tread to the tire radial modulus (a) thereof (i.e., b/a ratio) is adjusted to 1.2 or above by incorporating a short fiber (Axe2x80x2) formed by the fibrillation of a short fiber (A) whose cross section takes a sea-island structure essentially composed of at least two polymers into the tread rubber in a proportion of 0.5 to 15 parts by weight per 100 parts by weight of the rubber and orienting the short fiber (Axe2x80x2) in a tire circumferential direction, and the groove area ratio of the tread is adjusted to 30 to 40%
Another pneumatic tire of the present invention achieving the above third object is one wherein the ratio of the tire circumferential modulus (b) of the tread to the tire radial modulus (a) thereof (i.e., b/a ratio) is adjusted to 1.2 or above by incorporating a short fiber (Axe2x80x2) formed by the fibrillation of a short fiber (A) whose cross section takes a sea-island structure essentially composed of at least two polymers and a composition comprising a short fiber (B) made of a thermoplastic polymer having amido groups in the main chain and a matrix made of a rubber and/or a polyolefin wherein the short fiber (B) is dispersed in the matrix and chemically bonded to the matrix into the tread rubber in proportions of 0.5 to 10 parts by weight of the fibrillated short fiber (Axe2x80x2) and 1 to 15 parts by weight of the short fiber (B) per 100 parts by weight of the rubber and orienting the fibrillated short fiber (Axe2x80x2) and the short fiber (B) in a tire circumferential direction and the groove area ratio of the tread is adjusted to 30 to 40%.
Although a pneumatic tire having a larger ground-contacting area is more advantageous in braking performance on a dry road surface, it is also required to have sufficient groove area enough to secure satisfactory drainage performance on a wet road surface. Therefore, there is a limit to the improvement of both braking performance on a dry road surface and that on a wet road surface in a well-balanced state by changing the groove area ratio.
In the above cases wherein the groove area ratio of the tread is limited to the above range and such an anisotropic rubber tread that the tire circumferential modulus (b) is larger than the tire radial modulus (a) by a specific factor or above is employed, only the tire circumferential rigidity of the tread can selectively be enhanced. At the same time, sufficient actual ground-contacting area is secured by controlling the tire radial rigidity of the tread to a conventional level or below. Thus, the braking performance on a dry road surface can well meet with that on a wet road surface at high levels.
A pneumatic tire of the present invention achieving the above fourth object is one wherein the tread portion is constituted of at least two layers comprising a cap tread and an under tread, and the ratio of the tire circumferential modulus (b) of the under tread to the tire widthwise modulus (a) thereof (i.e., b/a ratio) is adjusted to 1.5 or above by incorporating a short fiber (Axe2x80x2) formed by the fibrillation of a short fiber (A) whose cross section takes a sea-island structure essentially composed of at least two polymers into the rubber constituting the under tread in a proportion of 1 to 15 parts by weight per 100 parts by weight of the rubber and orienting the fibrillated short fiber (Axe2x80x2) in a tire circumferential direction.
Another pneumatic tire of the present invention achieving the above fourth object is one wherein the tread is constituted of at least two layers comprising a cap tread and an under tread, and the ratio of the tire circumferential modulus (b) of the under tread to the tire widthwise modulus (a) thereof (i.e., b/a ratio) is adjusted to 1.5 or above by incorporating a short fiber (Axe2x80x2) formed by the.fibrillation of a short fiber (A) whose cross section takes a sea-island structure essentially composed of at least two polymers and a composition comprising a short fiber (B) made of a thermoplastic polymer having amido groups in the main chain and a matrix made of a rubber and/or a polyolefin wherein the short fiber (B) is dispersed in the matrix and chemically bonded to the matrix into the rubber constituting the under tread in proportions of 1 to 12 parts by weight of the fibrillated short fiber (Axe2x80x2) and 1 to 10 parts by weight of the short fiber (B) per 100 parts by weight of the rubber and orienting the fibrillated short fiber (Axe2x80x2) and the short fiber (B) in a tire circumferential direction.
For the purpose of meeting the recent demand for improved fuel consumption of vehicles, various attempts were made to reduce the rolling resistance of a tire. As a technique of reconciling improved fuel consumption with traveling performance on a wet road surface, for example, it has been a common practice to employ a two-layer tread constituted of a cap tread and an under tread to thereby decrease the volume of the cap tread exhibiting a large energy loss. In this practice, specifically, a rubber excellent in wet performances is used as the cap tread and a rubber exhibiting a small energy loss is used as the under tread.
When the content of carbon black in the under tread is lowered in order to lower the energy loss of the under tread, however, the resulting rubber gives too soft of a vulcanizate to attain satisfactory stability in steering. On the other hand, when a butadiene rubber excellent in impact resilience is incorporated into the under tread, the resulting under tread has such a low green viscosity as to cause rubber flow easily in pressing against the tread the groove-forming skeletons present on the inner surface of a mold during the molding of the tire with vulcanization. Therefore, the under tread becomes too thin in the portions located under grooves and becomes too thick in the portions located under blocks. Thus, the tire provided with an under tread containing a butadiene rubber or the like is disadvantageous in that the under tread comes out to the surface in the last stage of wear to result in poor traction performance.
In the above cases wherein the ratio of the tire circumferential modulus (b) of the under tread to the tire widthwise modulus (a) thereof (i.e., b/a ratio) is adjusted to 1.5 or above by incorporating a short fiber (Axe2x80x2) formed by the fibrillation of a short fiber (A) whose cross section takes a sea-island structure essentially composed of at least two polymers in a specific proportion and orienting the short fiber (Axe2x80x2) in a tire circumferential direction, the molding of the tire with vulcanization exhibits little rubber flow thereby obtained an improved uniformity in the under tread gauge, even if a rubber exhibiting a low energy loss is used as the under tread for the purpose of reducing the rolling resistance of the tire. After the vulcanization, the fibrillated short fiber (Axe2x80x2) is present in the under tread in a state oriented in a tire circumferential direction to exhibit a high reinforcing effect, thus improving the stability in steering.
The above fibrillated short fiber (Axe2x80x2) has excellent features as compared with the nylon short fiber and carbon black according to the prior art. Specifically, a rubber containing the fibrillated short fiber (Axe2x80x2) even in a large amount suffers little from the rise of tan xcex4, even though a rubber containing carbon black in an amount increased for the purpose of enhancing the hardness of the under tread exhibits an enhanced tan xcex4 with a rise in the hardness. Further, a rubber containing the fibrillated short fiber (Axe2x80x2) even in a large amount has a slightly increased green viscosity, even though a rubber containing a large amount of the nylon short fiber of the prior art exhibits too high a green viscosity to exhibit satisfactory processability in kneading and extrusion. Therefore, the incorporation of the fibrillated short fiber (Axe2x80x2) into a rubber can enhance the green modulus of the rubber not only in the grain direction (direction of orientation of the fiber) but also in the reverse grain direction (perpendicular to the direction of orientation of the fiber). Thus, the incorporation of the fibrillated short fiber (Axe2x80x2) into an under tread gives an under tread which is hard and exhibits a low heat build-up. Further, this under tread is therefore inhibited from rubber flow during the vulcanization of the tire by virtue of its high green strength, in spite of its being more excellent in processability than the one containing the nylon short fiber of the prior art. Furthermore, the incorporation of the fibrillated short fiber (Axe2x80x2) also brings about the effect of inhibiting the rubber compound from cold flow during the storage.
A pneumatic tire of the present invention achieving the above fifth object is one wherein at least a part of the sidewall is made of a rubber composition containing 0.5 to 15 parts by weight of a short fiber (Axe2x80x2) formed by the fibrillation of a short fiber (A) whose cross section takes a sea-island structure essentially composed of at least two polymer.
Another pneumatic tire of the present invention achieving the above fifth object is one wherein at least a part of the sidewall is made of a rubber composition containing a short fiber (Axe2x80x2) formed by the fibrillation of a short fiber (A) whose cross section takes a sea-island structure essentially composed of at least two polymers and a composition comprising a short fiber (B) made of a thermoplastic polymer having amido groups in the main chain and a matrix made of a rubber and/or a polyolefin wherein the short fiber (B) is dispersed in the matrix and chemically bonded to the matrix in proportions of 0.5 to 10 parts by weight of the fibrillated short fiber (Axe2x80x2) and 1 to 15 parts by weight of the short fiber (B) per 100 parts by weight of the rubber.
The resistance of a pneumatic tire to external damage is lowered, when the gauge of the sidewall rubber is merely decreased for the purpose of reducing the weight. Although a sidewall can be reinforced by adding nylon short fiber to the rubber constituting the sidewall, the reinforcement with nylon fiber results in an unsatisfactory reinforcing effect. Thus, no satisfactory resistance to external damage can be secured, when the weight of a tire is reduced by decreasing the sidewall gauge.
In the above cases wherein a short fiber (Axe2x80x2) formed by the fibrillation of a short fiber (A) whose cross section takes a structure essentially composed of at least two polymers is incorporated into the sidewall rubber in a specific proportion, the fibrillated short fiber (Axe2x80x2) enhances the modulus at low elongation by leaps and bounds to exhibit an excellent reinforcing effect, which makes it possible to secure satisfactory resistance to external damage even when the weight of a tire is reduced by decreasing the sidewall gauge.
It is preferable that the fibrillated short fiber (Axe2x80x2) be oriented in a direction parallel to the sidewall face. The fibrillated short fiber (Axe2x80x2) may be oriented in any state in the tire circumferential and radial directions, as far as it is oriented in a direction parallel to the sidewall face. For example, the short fiber (Axe2x80x2) may be oriented in a specific direction (such as circumferential direction) or arranged at random. The maximum reinforcing effect of the fibrillated short fiber (Axe2x80x2) can be exhibited, when the fiber (Axe2x80x2) is oriented in such a state.
A pneumatic tire of the present invention achieving the above sixth object is a heavy-duty pneumatic radial tire comprising a carcass layer which contains a plurality of carcass cords in a state arranged in the tire radial direction and bridges a pair of, i.e., lefthand and righthand bead portions with both tire widthwise end portions of the carcass layer wound up around the bead cores respectively from the tire inner side to the tire outer side, and which further comprises short fiber reinforced layers lying at least in the wound-up end portions of the carcass layer, with the ratio of the tire circumferential modulus (b) of the short fiber reinforced layers to the tire radial modulus (a) thereof (i.e., b/a ratio) being adjusted to 1.2 or above by incorporating a short fiber (Axe2x80x2) formed by the fibrillation of a short fiber (A) whose cross section takes a sea-island structure essentially composed of at least two polymers into the rubber constituting the short fiber reinforced layers in a proportion of 0.5 to 15 parts by weight per 100 parts by weight of the rubber and orienting the short fiber (Axe2x80x2) in a tire circumferential direction.
Another pneumatic tire of the present invention achieving the above sixth object is a heavy-duty pneumatic radial tire comprising a carcass layer which contains a plurality of carcass cords in a state arranged in the tire radial direction, bridges a pair of, i.e., lefthand and righthand bead portions with both tire widthwise end portions of the carcass layer wound up around the bead cores respectively from the tire inner side to the tire outer side, and further comprises short fiber reinforced layers lying at least in the wound-up end portions of the carcass layer, with the ratio of the tire circumferential modulus (b) of the short fiber reinforced layers to the tire radial modulus (a) thereof (i.e., b/a ratio) being adjusted to 1.2 or above by incorporating a short fiber (Axe2x80x2) formed by the fibrillation of a short fiber (A) whose cross section takes a sea-island structure essentially composed of at least two polymers and a composition comprising a short fiber (B) made of a thermoplastic polymer having amido groups in the main chain and a matrix made of a rubber and/or a polyolefin wherein the short fiber (B) is dispersed in the matrix and chemically bonded to the matrix into the rubber constituting the short fiber reinforced layers in proportions of 0.5 to 10 parts by weight of the fibrillated short fiber (Axe2x80x2) and 1 to 15 parts by weight of the short fiber (B) per 100 parts by weight of the rubber and orienting the fibrillated short fiber (Axe2x80x2) and the short fiber (B) in a tire circumferential direction.
There have been known heavy-duty pneumatic radial tires wherein reinforcing layers containing steel cords or organic fiber cords are laid in the bead portions for the purpose of enhancing durability. Even when such a reinforcing structure is formed, however, it is still difficult to inhibit the delamination among cords occurring in the wound-up end portions of the carcass layer.
Precisely, the wound-up end portions of the carcass layer contain parts wherein the carcass cords are not bonded to the rubber, so that cracks are initiated in such parts and then grown to result in the delamination among carcass cords. Although it has been known that the wound-up end portions of the carcass layer can be reinforced with rubber reinforcement layers, such reinforcement not only fails in attaining a satisfactory effect, but also brings about an increase in the weight.
In the above cases wherein such anisotropic short fiber reinforced rubber layers that the tire circumferential modulus (b) is larger that the tire radial modulus (b) by a specific factor or above are laid in the wound-up end portions of the carcass layer, the cords arranged in the tire radial direction are effectively inhibited from moving in a tire circumferential direction, so that the stress concentration in the wound-up end portions can be relaxed with a decrease in the rubber volume of the bead portions to thereby inhibit the delamination among cords and thus enhance the durability of the tire. Additionally, when a steel cord reinforced layer containing a plurality of steel cords in a state arranged in the tire radial direction is further laid in such a way as to lie along the carcass layer in the bead portions, the delamination among cords in the upper end portions of the steel cord reinforced layer can also be inhibited by laying the short fiber reinforced layers in the upper end portions of the steel cord reinforced layer.
A pneumatic tire of the present invention achieving the above seventh object is a pneumatic radial tire comprising bead fillers located on the outer peripheral sides of the bead cores respectively, wherein the bead fillers are made of a rubber composition containing a short fiber (Axe2x80x2) formed by the fibrillation of a short fiber (A) whose cross section takes a sea-island structure essentially composed of at least two polymers in a proportion of 3 to 15 parts by weight per 100 parts by weight of the rubber in a state oriented in a direction perpendicular to the tire radial direction, whereby the ratio of the storage modulus (b) of the bead fillers in the direction of orientation of the short fiber to the tire radial storage modulus (a) thereof (i.e., b/a ratio) is adjusted to 4 to 10.
Another pneumatic tire of the present invention achieving the above seventh object is a pneumatic radial tire comprising bead fillers located on the outer peripheral sides of the bead cores respectively, wherein the bead fillers are made of a rubber composition containing a short fiber (Axe2x80x2) formed by the fibrillation of a short fiber (A) whose cross section takes a sea-island structure essentially composed of at least two polymers and a composition comprising a short fiber (B) made of a thermoplastic polymer having amido groups in the main chain and a matrix made of a rubber and/or a polyolefin wherein the short fiber (B) is dispersed in the matrix and chemically bonded to the matrix in proportions of 3 to 10 parts by weight of the fibrillated short fiber (Axe2x80x2) and 1 to 15 parts by weight of the short fiber (B) per 100 parts by weight in a state oriented in a direction perpendicular to the tire radial direction, whereby the ratio of the storage modulus (b) of the bead fillers in the direction of orientation of the short fiber to the tire radial storage modulus (a) thereof (i.e., b/a ratio) is adjusted to 4 to 10.
In improving the stability in steering of a pneumatic radial tire having an aspect ratio of 60% or below, means for laying steel reinforcements in the neighborhood of the bead fillers is generally employed. This is because the rigidity of the bead portions cannot sufficiently be enhanced merely by enhancing the rigidity of the bead fillers. When such steel reinforcements are laid in the bead portions, however, the resulting tire is significantly deteriorated in comfortableness in riding owing to its too high tire radial rigidity and is increased in weight. On the other hand, when no steel reinforcement is used, the resulting tire is poor in stability in steering, because of the bead portions being too soft.
In the above cases wherein anisotropic rubber bead fillers which contain the fibrillated short fiber (Axe2x80x2) in a state oriented in a direction perpendicular to the tire radial direction and in which the storage modulus (b) in the direction of orientation of the short fiber is larger than the tire radial storage modulus (a) by a specific factor or above are laid on the outer peripheral sides of the bead cores, the rigidity of the bead portions in a direction perpendicular to the tire radial direction can be enhanced without any substantial change in the rigidity of the bead portions in the tire radial direction. Accordingly, the use of such bead fillers makes it possible to secure not only stability in steering equivalent or superior to that of a tire provided with steel reinforcements in the neighborhood of the bead fillers but also comfortableness in riding and a light weight equivalent to those of a tire not provided with any steel reinforcement.
The above anisotropy of the bead fillers can be attained by orienting the fibrillated short fiber (Axe2x80x2) in a direction perpendicular to the tire radial direction. The short fiber (Axe2x80x2) may be oriented in a tire circumferential direction or the tire rotating shaft direction. The orientation in a tire circumferential direction is preferable. Alternatively, two-layer bead fillers constituted of two layers laminated in the tire rotating shaft direction may be employed, one of the layers containing the short fiber in a state oriented in the tire circumferential direction and the other layer containing it in a state oriented in the tire rotating shaft direction.
It is preferable that the above anisotropic constitution of bead fillers be applied to pneumatic radial tires having aspect ratios of 60% or below. The application to such oblate tires brings about more remarkable functions and effects.
In this specification, each storage modulus value is one determined with respect to a test piece having a width of 5 mm, a length of 20 mm and a thickness of 2 mm by using a viscoelasticity spectrometer at a strain of 5xc2x11%, a frequency of 20 Hz and a temperature of 20xc2x0 C.